1. Field of the Invention
This invention relates to a high-pressure discharge lamp, a turning-on circuit device for the high-pressure discharge lamp, and a lighting fixture provided with the high-pressure discharge lamp and turning-on circuit device.
2. Description of the Related Art
To turn on a high-pressure metal-vapor discharge lamp, such as a high-pressure sodium lamp or a metal halide lamp, again after turning off the lamp, it is necessary to wait for the luminescent tube to get cool to such an extent that the vapor pressure of the mercury and sealed luminescent metal in the tube drops, causing the internal pressure to fall below a specific level, because the vapor pressure of the mercury, sealed luminescent metal, and starting gas is higher than normal atmospheric pressure during lighting. For instance, in the case of a high-pressure sodium lamp, the external start type has to wait for about one minute and the internal start type has to wait for about ten minutes as restart times. In the case of a metal halide, it is necessary to wait for as long as more than ten minutes.
Therefore, for example, when electric power fails for a moment, an incandescent lamp or a fluorescent lamp lights up again immediately, whereas it takes longer than ten minutes for the high-pressure metal vapor discharge lamp to recover the full power. Thus, if the high-pressure metal vapor discharge lamps were used to illuminate a road or the inside of a tunnel, it is inconvenient for illumination and also may have a safety problem.
To overcome these disadvantages, a high-pressure sodium lamp designed to house two luminescent tubes in the external tube, electrically connect the two tubes in parallel with each other, and turn on one of the luminescent tubes, has been disclosed in U.S. Pat. No. 4,287,454.
The luminescent tubes in such a high-pressure sodium lamp are supported in such a manner that the two luminescent tubes are arranged in parallel, the both ends of the tubes are supported by support plates, a support rod is inserted between the support plates so as to be in parallel with the luminescent tubes, and the support rod is supported by the stem of the external tube.
This type of high-pressure lamp, however, has a layout where the two parallel luminescent tubes overlap greatly with each other, so that the shade of the adjacent luminescent tube is large during lighting, which decreases the amount of light so much more, affecting the performance of the lamp.
To avoid the overlapping of the luminescent tubes, means for providing two luminescent tubes inclined to the vertical direction so as to cross each other in an X shape has been disclosed in Jpn. Pat. Appln. KOKAI Publication No. 4-218255.
Crossing the two luminescent tubes in an X shape reduces the shade caused by the unlighted luminescent tube, which improves the nonuniformity of light distribution around the lamp axis remarkably. Some lamps of this type, however, undergo a decrease in the luminous flux.
A study of the cause of a decrease in the luminous flux has revealed that although in the discharge lamp, the temperature is generally the highest in the middle of the luminescent tube during lighting, crossing the two lamps in an X shape makes the temperature in the middle of the tube much higher. The reason is that the radiant heat from the lighted luminescent tube hits the other unlighted luminescent tube made of white ceramic and is reflected by the unlighted tube, and then returns to the lighted luminescent tube. The middle of the tube is at high temperatures from the beginning and tends to be at extremely high temperatures because the two luminescent tubes are close to each other. The study also has shown that when the luminescent tube is at extremely high temperatures, the sodium sealed in the luminescent tube as luminescent metal reacts with aluminum, a material forming the luminescent tube bulb, and disappears, with the result that lack of sodium leads to a decrease in the luminous flux.